1. Field of the Invention
The present invention relates to a film forming apparatus for forming a crystalline silicon film, which is used, e.g., as a material of a TFT (Thin-Film Transistor) switch provided for each pixel in a liquid crystal display, an integrated circuit, a solar battery and others, and for forming, if necessary, an electrical insulating film such as a silicon compound film in addition to the crystalline film.
The present invention also relates to a method of forming a crystalline silicon film.
2. Description of the Background Art
Amorphous silicon films have been used as semiconductor films for TFTs and others because amorphous silicon films having large areas can be made at a low temperature. However, it is now desired to provide crystalline silicon films having a crystal grain diameter of 200 nm or more and, particularly, about 300 nm or more for improving transistor characteristics and forming devices integrally provided with drive circuits.
The crystalline silicon film can be formed by several methods. For example, it may be formed by a thermal CVD method in which a substrate, i.e., a work or an object on which the film is to be formed or deposited, is heated to a high temperature of 600.degree. C. or more, and the film is formed by the thermal CVD under normal or reduced pressure. In another method, the crystalline silicon film is formed by a PVD method such as a vacuum deposition or sputter deposition while keeping the substrate at a temperature of about 700.degree. C. or more. In a still another method, an amorphous silicon film is formed by appropriate CVD or PVD at a relatively low temperature, and then is crystallized by post-treatment, which is heat treatment effected on the amorphous silicon film at a temperature of 800.degree. C. or more, or is heat treatment effected thereon at a temperature of about 600.degree. C. for a long time of about 20 hours or more. Further, such a method may be employed that laser annealing is effected on an amorphous silicon film for crystallizing the same.
Among these methods, the laser anneal method, in which the laser annealing is effected on the amorphous silicon film for crystallizing the same, can produce the crystalline silicon film at a lower temperature than the other methods. Therefore, substrates to be processed in the laser anneal method are not restricted to materials such as quartz having a high melting point, and the method can be employed for materials such as glass which are relatively inexpensive and have a low melting point. Further, the laser annealing itself can be performed within a relatively short time, which improves an efficiency of formation of the crystalline silicon films.
However, formation of the amorphous silicon film and the laser annealing are usually performed in different apparatuses, respectively, and the substrate coated with the film is once moved into an atmosphere for bringing the substrate and film into a laser irradiation apparatus.
In some cases, an electrical insulating film such as a silicon compound film is formed before or after formation of the amorphous silicon film. The insulating film is usually formed in another apparatus. Therefore, it is difficult to keep clean boundaries or interfaces between the respective films, and therefore it is difficult to provide good device characteristics. In addition, long times are required for transportation of the substrate between the apparatuses and repetitive heating of the substrate, resulting in a low throughput.
According to the method in which the amorphous silicon film is irradiated with a laser beam, the laser must have a very high energy density for providing the crystalline silicon film having a crystal grain diameter of a practically required value of 200 nm or more and, more preferably, a value of 300 nm or more. For this, the laser irradiation apparatus must have a high power. Accordingly, an expensive apparatus is required, and the laser beam emitted with a high power becomes unstable, resulting in reduction in productivity.
Since the amorphous silicon film contains a large amount of hydrogen mixed thereinto, the quality of the film would be impaired due to bumping of hydrogen if the film were irradiated with the laser beam as it was. Therefore, heat treatment must be effected on the amorphous silicon film formed on the substrate for removing the hydrogen, and thus time-consuming processing is required.
Many dangling bonds are present in the crystalline silicon film which is formed by crystallizing the amorphous silicon film. For obtaining good device characteristics, therefore, the dangling bonds must be filled with hydrogen by exposing the crystalline silicon film to hydrogen plasma, which also requires time consuming processing.
In the method of crystallizing the amorphous silicon film by laser beam irradiation, the laser must have an extremely high energy density for producing the crystalline silicon film having a crystal grain diameter of a practically allowable value, as already described. However, the vacuum deposition, sputter deposition and others cannot form the amorphous silicon films having a sufficiently high adhesion to the substrate. Therefore, local separation of the film is liable to occur due to a stress which occurs in the film during the laser annealing.
In the case where the amorphous silicon film is formed by the vacuum deposition, sputter deposition or the like, it is difficult to control the grain diameter of the crystal which is formed by the later crystallizing processing. Therefore, the finally produced film may not have a sufficiently large crystal grain diameter, or may have an excessively large grain diameter, resulting in irregularities on grain boundaries and thus a large surface roughness of the film.